FINAL TECHNICAL MEMORANDUM
EXTREME RAINFALL ANALYSIS
EXECUTIVE SUMMARY
EXTREME RAINFALL ANALYSIS
This report documents an extreme rainfall analysis conducted as part of the Fargo-Moorhead Metro Area
Diversion Project (FM Diversion). The analysis was completed to determine the magnitude of flooding
that could be expected from an extreme rainfall event in the Red River Valley, and to identify how the FM
Diversion would operate under these extreme conditions. In addition to the extreme rainfall events
analyzed, this study evaluates the potential effects of the precipitation that fell during the 2009 spring
flood event on the peak flood stage at USGS Gage 05054000, Red River at Fargo.
Several previously developed modeling tools were utilized and expanded upon for this analysis.
Hydrologic Engineering Center – Hydrologic Modeling System (HEC-HMS) models were previously
developed for watersheds within the Red River Basin upstream from the City of Halstad, MN for the
“Fargo-Moorhead Metro Basin-Wide Modeling Approach” report, December, 2011. This study is also
referred to as the Phase 1 Basin-Wide Modeling Effort.
To generate the extreme rainfall inflow hydrographs for the unsteady HEC-RAS model, Next-Generation
Radar (NEXRAD) data was obtained from the National Weather Service (NWS) and the National Oceanic
and Atmospheric Administration (NOAA). The NEXRAD data was obtained for three historic storms from
within the Upper Midwest. The storms include:
a. 2004 Southern Minnesota and Northern Iowa – Southern Minnesota and Northern Iowa
received heavy rainfall on September 14 and 15, 2004 with totals exceeding 12 inches over a
span of approximately 30 hours.
b. 2007 Southeast Minnesota – A wide area of Northern Iowa and Southern Minnesota
received over 6 inches of rainfall with areas of Southeastern Minnesota receiving over 15
inches throughout a five day period.
c. 2008 Northern Iowa – Several rounds of intense rain fell over Iowa, Wisconsin and
Southeastern Minnesota. The hardest hit areas received nearly 10 inches of rainfall.
The rainfall data was manipulated using several Geographic Information System (GIS) tools. It was then
transferred to various locations within the upper portion of the Red River Valley. Several storm
orientations were evaluated in order to identify the most significant resulting peak flow on the Red River
at Fargo.
Flows from the NEXRAD rainfall data were inserted into the HEC-HMS models. Resulting hydrographs
from the HEC-HMS models were extracted and placed in the Hydrologic Engineering Center – River
Analysis System (HEC-RAS) model used for the FM Diversion Project. The unsteady HEC-RAS model was
executed for existing conditions to determine the maximum peak flow, and the FM Diversion model to
identify how the project would operate for the extreme flows. The current FM Diversion model is VE-13
Option A (VE13A) with Bundles, which includes passing more water through the Flood Damage Reduction
Area (FDRA) and operable gates on the diversion inlet.
It was found that an extreme rainfall event distributed upstream of the project area, such as those
evaluated, is not likely to produce discharges greater than that of a 0.2-percent chance event on the Red
River. In addition, the VE13A project operation was set to produce resulting downstream impacts that
were equal to or less than the impacts identified for the 1-percent chance event from the Integrated Final
Feasibility Report and the Final Environmental Impact Statement (FR/FEIS) dated July 2011. The resulting
upstream staging area elevations, stage through town, and downstream impacts for the 2004, 2007, and
2008 events are reasonably comparable to the current FM Diversion design events.
Although the exact FM Diversion operation implemented for this analysis could be modified to produce
more or less downstream impacts resulting in different upstream staging elevations, the analysis
concludes that the proposed project is capable of effectively operating during the extreme rainfall events
evaluated in this analysis.
PRECIPITATION DURING THE SPRING 2009 FLOOD EVENT
In addition to the extreme rainfall events analyzed, this study evaluates the potential effects of the
precipitation that fell during the 2009 spring flood event on the peak flood stage at USGS Gage 05054000,
Red River at Fargo if the precipitation fell as rain. Much of the precipitation fell as snow because of the
cold temperatures leading up to the flood crest. The objective of this analysis was to determine what the
additional peak discharge and resulting stage would be at Fargo if the snowfall precipitation fell as rain
instead.
Two snowfall events occurred during the 2009 spring flood. The first one produced an average of 0.5
inches of Snow Water Equivalent (SWE) over much of the basin, while the second one contributed up to
2.5 inches of SWE. The spatially distributed SWE data was placed into the HEC-HMS and the resulting
hydrographs inserted in the HEC-RAS models in a similar fashion as in the extreme rainfall analysis.
The increase in discharge from the 0.5” water equivalent rainfall on March 25 was approximately 500 cfs
at the flood crest (March 28). The rainfall equivalent from the event occurring between March 30 and
April 1 was more heavily centered on Wahpeton and the southern portion of the Valley. This produced an
increase in discharge of approximately 8,000 cfs centered on April 8th. However, since this event
occurred after the flood crest, the additional discharge would not have increased the overall crest of the
flood.
If the rainfall would have aligned with the crest at Fargo, it is anticipated that the increase in discharge
would have been less than 8,000 cfs because the River’s discharge-elevation relationship varies and a
higher stage would introduce additional natural floodplain storage and higher tailwater. This also makes
it difficult to predict an actual stage increase as a result of the rainfall.
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